feat(tui): add LIGHT_THEME preset for white/light terminal backgrounds

Splits the existing palette into DARK_THEME (current yellow-heavy
default) and LIGHT_THEME (darker browns + proper contrast on white).
DEFAULT_THEME aliases DARK_THEME, and flips to LIGHT_THEME when
HERMES_TUI_LIGHT=1 is set at launch.

Skin system (fromSkin) still layers on top of whichever preset is
active, so users can keep customizing on top of either palette.

Refs #11300.

ui-tui/src/__tests__/theme.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 
-import { DEFAULT_THEME, fromSkin } from '../theme.js'
+import { DARK_THEME, DEFAULT_THEME, fromSkin, LIGHT_THEME } from '../theme.js'
 
 describe('DEFAULT_THEME', () => {
   it('has brand defaults', () => {
@@ -15,6 +15,26 @@ describe('DEFAULT_THEME', () => {
   })
 })
 
+describe('LIGHT_THEME', () => {
+  it('avoids bright-yellow accents unreadable on white backgrounds (#11300)', () => {
+    expect(LIGHT_THEME.color.gold).not.toBe('#FFD700')
+    expect(LIGHT_THEME.color.amber).not.toBe('#FFBF00')
+    expect(LIGHT_THEME.color.dim).not.toBe('#B8860B')
+    expect(LIGHT_THEME.color.statusWarn).not.toBe('#FFD700')
+  })
+
+  it('keeps the same shape as DARK_THEME', () => {
+    expect(Object.keys(LIGHT_THEME.color).sort()).toEqual(Object.keys(DARK_THEME.color).sort())
+    expect(LIGHT_THEME.brand).toEqual(DARK_THEME.brand)
+  })
+})
+
+describe('DEFAULT_THEME aliasing', () => {
+  it('defaults to DARK_THEME when HERMES_TUI_LIGHT is unset', () => {
+    expect(DEFAULT_THEME).toBe(DARK_THEME)
+  })
+})
+
 describe('fromSkin', () => {
   it('overrides banner colors', () => {
     expect(fromSkin({ banner_title: '#FF0000' }, {}).color.gold).toBe('#FF0000')

ui-tui/src/theme.ts

@@ -78,7 +78,17 @@ function mix(a: string, b: string, t: number) {
 
 // ── Defaults ─────────────────────────────────────────────────────────
 
-export const DEFAULT_THEME: Theme = {
+const BRAND: ThemeBrand = {
+  name: 'Hermes Agent',
+  icon: '⚕',
+  prompt: '❯',
+  welcome: 'Type your message or /help for commands.',
+  goodbye: 'Goodbye! ⚕',
+  tool: '┊',
+  helpHeader: '(^_^)? Commands'
+}
+
+export const DARK_THEME: Theme = {
   color: {
     gold: '#FFD700',
     amber: '#FFBF00',
@@ -112,20 +122,59 @@ export const DEFAULT_THEME: Theme = {
     shellDollar: '#4dabf7'
   },
 
-  brand: {
+  brand: BRAND,
-    name: 'Hermes Agent',
-    icon: '⚕',
-    prompt: '❯',
-    welcome: 'Type your message or /help for commands.',
-    goodbye: 'Goodbye! ⚕',
-    tool: '┊',
-    helpHeader: '(^_^)? Commands'
-  },
 
   bannerLogo: '',
   bannerHero: ''
 }
 
+// Light-terminal palette: darker golds/ambers that stay legible on white
+// backgrounds. Same shape as DARK_THEME so `fromSkin` still layers on top
+// cleanly (#11300).
+export const LIGHT_THEME: Theme = {
+  color: {
+    gold: '#8B6914',
+    amber: '#A0651C',
+    bronze: '#7A4F1F',
+    cornsilk: '#3D2F13',
+    dim: '#7A5A0F',
+    completionBg: '#F5F5F5',
+    completionCurrentBg: mix('#F5F5F5', '#A0651C', 0.25),
+
+    label: '#7A5A0F',
+    ok: '#2E7D32',
+    error: '#C62828',
+    warn: '#E65100',
+
+    prompt: '#2B2014',
+    sessionLabel: '#7A5A0F',
+    sessionBorder: '#7A5A0F',
+
+    statusBg: '#F5F5F5',
+    statusFg: '#333333',
+    statusGood: '#2E7D32',
+    statusWarn: '#8B6914',
+    statusBad: '#D84315',
+    statusCritical: '#B71C1C',
+    selectionBg: '#D4E4F7',
+
+    diffAdded: 'rgb(200,240,200)',
+    diffRemoved: 'rgb(240,200,200)',
+    diffAddedWord: 'rgb(27,94,32)',
+    diffRemovedWord: 'rgb(183,28,28)',
+    shellDollar: '#1565C0'
+  },
+
+  brand: BRAND,
+
+  bannerLogo: '',
+  bannerHero: ''
+}
+
+const LIGHT_MODE = /^(?:1|true|yes|on)$/i.test((process.env.HERMES_TUI_LIGHT ?? '').trim())
+
+export const DEFAULT_THEME: Theme = LIGHT_MODE ? LIGHT_THEME : DARK_THEME
+
 // ── Skin → Theme ─────────────────────────────────────────────────────
 
 export function fromSkin(